It has been common in the past, in the case of serious bone fractures in which simple resetting is contraindicated, to replace all or part of the bone with a suitable implant generally made of titanium or another metal. An example of an implant of this type is a femoral implant which includes a stem for insertion in the intermedullary canal of the femur, a neck portion at an angle to the stem, and a capitulum carried at the end of the neck. Because of the shape of the prosthesis and the high-stress location in which it is employed, its outer or lateral surface is subjected to substantial bending stresses, putting the outer surface in tension, during each step that a patient takes. Since titanium can withstand repeated cycles (i.e. 10.sup.8 cycles) of stress of about 80,000 psi, smooth titanium implants are generally used for this type of application.
It is often preferable to render porous the outer surfaces of the prosthesis by applying a thin coating of metallic beads to enhance the ingrowth of bone material. The porous metallic bead coatings are generally sintered onto the surface of the prosthesis utilizing relatively high temperatures. The combination of the high temperature and the application of the beads causes some of the beads to penetrate the surface of the prosthesis forming notches or stress-concentration sites. These stress concentration sites are likely locations from which cracks may form in the outer surface of the prosthesis as it is continuously cycled in tension. Cracks are very undesirable because they can significantly weaken the prosthesis and can cause the prosthesis to fail.
The characteristics of the surface of the prosthesis affects the amount of stress that the prosthesis can withstand at a given number of stressing cycles. The amount of stress that a prosthesis can withstand, while staying within its elastic limit, at a large number of cycles of stress (e.g., 10.sup.8 cycles) is referred to as the fatigue endurance limit. Titanium and titanium alloys are particularly notch-sensitive in that when prostheses formed of these materials are notched in stressed areas, the fatigue endurance limit may be significantly reduced.
It is preferrable to have bone implant prostheses having porous coatings because bone ingrowth into the coatings can significantly strengthen the bone-prosthesis bond. However, it has been found that prostheses of the type described above are prone to premature failure due to the abundant stress concentration sites and their tendency to promote cracking. In general, prostheses with porous outer surfaces can withstand repeated (e.g., 10.sup.8) tensile stress cycles of only about 20,000 psi.
The metallic bead coatings are further preferred because they have elastic qualities that are somewhat similar to those of the metal upon which they are placed. This feature enables both the metal implant and the thin coating to stretch and flex together without interrupting the bond between them.
Ceramic porous coatings are also used, but since ceramic materials do not possess good elastic characteristics, slight bending of the femoral implant (such as that caused by normal walking) may crack and loosen the ceramic beaded surface.
It is desired to have a bone implant prosthesis with a porous outer surface possessing the strength to withstand the repeated stresses of normal implanted use.